Doctor blades, such as chambered doctor blades, are extensively used in rotary-printing units, especially flexoprinting units, for applying ink, lacquer, adhesive or the like to a rotatable cylinder included in the printing unit. In a flexoprinting unit, for example, the chambered doctor blade serves to ink the screen roller. This occurs by filling the cells or recesses of the roller with ink by means of the chambered doctor blade. Such a chambered doctor blade is disclosed, for example, in International Application No. WO 93/24328. Chambered doctor blades of this type comprise an elongated frame with two elongated doctor blades, which are arranged alongside the roller in such a manner that the longitudinal axis of the chambered doctor blade defines an elongated chamber, which holds, for example, ink. When applying ink or the like to the circumferential surface of the roller, the chambered doctor blade is applied against the same. The function of each doctor blade changes with the direction of rotation of the roller. One of the doctor blades, namely the wiping doctor blade, wipes off excess ink, while the other merely has a sealing function.
The inking of the screen roller is essential to the printing process. It is of special importance that the screen roller is uniformly inked, which means that the distance between the roller and the doctor blades has to be accurately set. As a result, the chambered doctor blade, usually clamped in the machine frame of the printing unit, must exhibit good flexural and torsional rigidity.
Prior art chambered doctor blades therefore comprise a sturdy frame, which is usually solid and made in one piece, and on which the doctor blades are mounted. European Patent Application No. A-0 350 839 and International Application No. WO-A-89/07047, for instance, teach chambered doctor blades of this type.
Conventionally, such doctor blade frames are made of solid cast iron or aluminium, or compression-molded blanks of iron or aluminium provided with stiffening springs to reduce weight. However, these known constructions suffer from some serious drawbacks. First, the chambered doctor blade becomes very heavy and unwieldy and is thus difficult to dismount from the printing unit, e.g. when it is to be cleaned or replaced. It usually takes two people to dismount a cast-iron chambered doctor blade. Second, one casting mould is needed for each length of chambered doctor blade. Third, the cast-iron structure is naturally susceptible to corrosion, which constitutes a serious inconvenience, since the printing ink to be circulated in the chamber often contains corrosive components.
The market also provides doctor blade frames consisting of extruded aluminium sections in one piece, but these do not offer any satisfactory solution to the above problems. In order to withstand the contemplated strains and to obtain sufficient flexural and torsional rigidity, the aluminium sections must be comparatively thick, and the chambered doctor blade will thus become unnecessarily heavy. Furthermore, the aluminium sections are also liable to corrosion, since the printing ink often contains basic substances which are aggressive to aluminium.
In addition to the requirements discussed in the foregoing, the ink chamber naturally has to be sealed. The doctor blade that removes excess ink for the contemplated direction of rotation of the roller, is the operative doctor blade, and the other doctor blade merely has a sealing function. When the direction of rotation is reversed, it naturally is the other way around. The two doctor blades have to be applied against the circumferential surface of the screen roller in precisely the right way for the ink to be evenly distributed on the roller and to minimise the amount of ink dropping from the lower doctor blade (when this is the sealing blade). Moreover, special seals are required at each end of the chamber in this respect. Reference is made to U.S. Pat. No. 4,581,995, which teaches a sealing unit placed at the end of an ink chamber and consisting of a pressure and labyrinth seal made up of several thin sealing lamellae of polymeric material.
U.S. Pat. No. 5,671,673 teaches a chambered doctor blade device where the frame of the cambered doctor blade, in order to achieve a good stiffening and reinforcing effect, is composed of an assembly of interconnected metal sections. The frame composed of metal sections should form a flexurally and torsionally rigid unit which ensures a contact distance between the frame and the cylinder against which the doctor blades are to be applied. With such a design on the frame, the whole chambered doctor blade can be of much higher and more slender construction, without lowering the standards of strength.
The chambered doctor blade device, according to U.S. Pat. No. 5,671,673, with an internal chamber having clamping strips, strip-blade holder or single doctor blades, channels, as well as an external chamber, has too many parts, unnecessary inked surfaces and nooks with capillary slots which are difficult to access in order to achieve simple and effective cleaning of the printing unit.
A general problem with prior art doctor blade systems is that the doctor blades and the cylinder against which the blades are applied are worn out to quickly, which is costly and causes frequent changes of the blades and cylinders, and frequent cleaning operations.
Another problem which occurs in doctor blade systems is that when the doctor blade clamping portion breaks, the whole doctor blade system must be replaced or sent for repair.
One object of the present invention is to provide a doctor blade system which, despite its low weight, has sufficient flexural and torsional rigidity to ensure that a rotating cylinder is evenly covered, so that the doctor blade has an even contact against the cylinder.
Another object of the present invention is to provide a doctor blade system that is easy to clean and maintain.
A further object of the present invention is to provide a doctor blade system in which the stress on the doctor blade as well as the cells and cell walls of the rotating cylinder decreases.
A still further object of the present invention is to provide a doctor blade system which facilitates simple, safe and fast change of doctor blades and end seals in and outside the printing press.
A still further object of the present invention is to provide a doctor blade system which facilitates simple and quick change of ink without having to remove the chamber from the printing press.
Yet another object of the present invention is to provide a chambered doctor blade system which relatively easy can be renovated.